I will fly to Slovenia from Noibai Airport, Vietnam. I have Schengen Visa already. As I will have to transit at Moscow airport for 15 hour, I would like to apply for visa on arrival to exit the airport and visit your beautiful capital. Is it possible as I already got the Schengen visa?
If you are a citizen of Vietnam then you will need a visa to visit Russia, even for less than one day. There is no visa on arrival; you must obtain the visa in advance. Having a Schengen visa is of no help; some countries will allow Schengen visa holders to enter without a visa, but Russia is not one of them. (You don't need a visa to visit Russia if you have an APEC Business Travel Card endorsed RUS, but you would presumably already know this if you had such a card.)
To obtain a visa, you can fill out the official Russia visa application online, and then take the application and your required documents to the VFS Global office in Hanoi, Ho Chi Minh City or Danang.
You don't need a visa if you remain in the international transit area of the airport. There are several hotels and capsule hotels in the international transit areas of SVO in terminals D and E departure areas. You will be able to sleep in one of these hotels if you wish, and do not need to clear immigration.